Apr 28 2017
In futuristic science fiction films like "Minority Report" and "Iron Man," characters are seen controlling computer displays with slick and deliberate hand motions. For instance, the protagonist in "Minority Report” played by Tom Cruise uses gloves with fingertips that glow to provide him the power of virtual manipulation. It looks like the light allows him to control the screen as if it was a touchscreen, but he is just touching air and nothing else.
Although that technology still remains as science fiction, a new study seeks to bring it closer to reality. Researchers from Japan report this week in Applied Physics Letters, from AIP Publishing, that they have found a phenomenon known as the photodielectric effect, which could pave the way for laser-controlled touch displays.
A number of rudimentary circuit parts have been created beyond their traditional electricity-based designs to instead be manipulated with light, such as photodiodes, photo-resistors, and phototransistors. However, there is still no photo-capacitor.
A photo-capacitor provides a novel way for operating electronic devices with light. It will push the evolution of electronics to next-generation photo-electronics.
Hiroki Taniguchi of the University of Nagoya in Japan
Capacitors are standard components for a variety of electronics, acting like buckets for electrons that can, for instance, filter unwanted frequencies or store energy. Most simply, a capacitor has two parallel conducting plates divided by an electrically insulating material, called a dielectric, such as glass or air. Applying a voltage across the plates causes opposing (and equal) charges to pile up on both plates.
The dielectric's properties have a determinate role to play in the electric field profile between the plates and, in turn, how much energy the capacitor can store. By using light to boost a property of the dielectric called permittivity, Taniguchi and his colleagues expect to develop light-controlled capacitors.
Earlier researchers have accomplished a type of photo-dielectric effect using many types of materials, but depended upon photo-conductance, where light increased the materials electrical conductivity. The rise in conductance, it turns out, caused greater dielectric permittivity.
But this type of extrinsic photodielectric effect is not ideal for practical applications, Taniguchi said. A capacitor has to be a good insulator, stopping electrical current from flowing. However under the extrinsic photodielectric effect, a capacitor's insulating properties depreciate. Furthermore, such a capacitor would only function with low-frequency alternating current.
Recently, Taniguchi and his colleagues have discovered an intrinsic photodielectric effect in a ceramic with the composition LaAl9.9Zn0.01O3-δ.
We have demonstrated the existence of the photodielectric effect experimentally.
Taniguchi
During their experiments, they shined an LED onto the ceramic and measured its dielectric permittivity, which amplified even at high frequencies. But unlike previous experiments that used the extrinsic photodielectric effect, the material continued to be a good insulator.
The lack of a substantial loss means the LED is directly changing the dielectric permittivity of the material, and, specifically, is not raising conductance, as is the case with the extrinsic effect. It is still uncertain how the intrinsic photodielectric effect operates, Taniguchi said, but it may be linked to the defects in the material.
Light stimulates electrons into higher (quantized) energy states, but the quantum states of defects are restricted to smaller regions, which may be stopping these photo-excited electrons from moving sufficiently far to produce an electric current. The hypothesis being that the electrons stayed trapped which causes more electrical insulation of the dielectric material.
Further research is necessary before light-controlled screens are a reality, but the research is an important step for the field. Going forward the research will seek to improve the effect even more, reduce any energy dissipation because of a drop of dielectric properties, and enhance the material fabrication process, Taniguchi said. More studies may also bring to light new materials better suited for other electronics applications.